FIG. 22 shows a conventional overcurrent detection circuit 12 disclosed in, for example, U.S. Pat. No. 6,011,413 corresponding to JP-3680513. The overcurrent detection circuit 12 detects an overcurrent (i.e., excessive current) condition in an N-channel metal oxide semiconductor (MOS) transistor 2 that is connected in series with an electrical load 1 to drive the load 1 by a load current IL. For example, the load 1 is a solenoid, a lamp, or a DC motor of a vehicle. The series circuit of the load 1 and the MOS transistor 2 is connected between a power supply voltage VB and a first ground PGND. Each of the MOS transistor 2 and another N-channel MOS transistor 3 has a source connected to the first ground PGND and a gate connected to an output of a driver circuit 4. Thus, the MOS transistors 2, 3 are turned on/off at the same time by a gate signal IN applied to an input of the driver circuit 4.
The MOS transistor 2 has a drain connected to a non-inverting input of an operational amplifier (op-amp) 5 and the MOS transistor 3 has a drain connected to an inverting input of the op-amp 5. An output of the op-amp 5 is connected to a terminal a of a current mirror circuit 6. The current mirror circuit 6 includes three PNP transistors 7-9. Each of the transistors 7, 8 has an emitter connected to the output of the op-amp 5 and has a base connected to an emitter of the transistor 9. The transistor 9 has a base connected to a collector of the transistor 8 and a collector connected to a second ground GND.
The transistor 7 has a collector (i.e., a terminal c of the current mirror circuit 6) connected to the second ground GND via a resistor 10 for detecting a collector current I1 of the transistor 7. The collector (i.e., a terminal b of the current mirror circuit 6) of the transistor 8 is connected to the drain of the MOS transistor 3. The collector of the transistor 7 is connected to an input of a voltage sensor 11. A size ratio of the MOS transistor 2 to the MOS transistor 3 is set to N to 1, where N is a positive number.
The overcurrent detection circuit 12 operates as follows.
When the MOS transistors 2, 3 are turned on at the same time by the gate signal IN applied to the input of the driver circuit 4, the load current IL flows through the load 1 and the MOS transistor 2. An output voltage of the op-amp 5 is controlled such that a drain voltage of the MOS transistor 2 is equal to that of the MOS transistor 3. As a result, a current ratio of the load current IL to a current IM flowing through the MOS transistor 3 is N to 1. For example, when the size ratio of the MOS transistor 2 to the MOS transistor 3 is set to 10 to 1, the current IM is ten times smaller than the load current IL.
The current mirror circuit 6 copies the input current to the output so that the collector current I1 can be equal to the current IM. The collector current I1 flows through the resistor 10 and a voltage is produced across the resistor 10. The voltage sensor 11 determines, based on the voltage, whether the overcurrent condition occurs in the MOS transistor 2. For example, when the voltage exceeds a predetermined threshold voltage, the voltage sensor 11 outputs an overcurrent detection signal IV. Alternatively, the voltage sensor 11 converts the voltage to a digital signal and a central processing unit (CPU) of an external circuit determines, based on the digital signal, whether the overcurrent condition occurs in the MOS transistor 2.
However, as discussed below, the overcurrent detection circuit 12 has a problem to be overcome. A collector voltage Vc(T8) of the transistor 8 is given by:Vc(T8)=Ve(T8)−Vbe(T8)−Vbe(T9)  (1)
In the above equation (1), Ve(T8) represents an emitter potential of the transistor 8, Vbe(T8) represents a base-emitter voltage of the transistor 8, and Vbe(T9) represents a base-emitter voltage of the transistor 9.
A collector potential Vc(T7) of the transistor 7 is given by:Vc(T7)=R10×I1  (2)
In the above equation (2), R10 represents a resistance of the resistor 10.
As can be seen by comparing the equations (1) and (2), the collector potential Vc(T8) is different from the collector potential Vc(T7). Therefore, an emitter-collector voltage of the transistor 8 is different from that of the transistor 7 and a difference between the collector current I1 and the current IM is caused by an early effect.
As described above, since the current mirror circuit 6 operates incorrectly due to the early effect, an accuracy of the overcurrent detection circuit 12 to detect the overcurrent condition is reduced.